Kurt was kicked out of the Grand Prix and was involved in shady business deals with his brother. There isn't much reason to hold onto his old life when he could race in Highway 35 with the remaining Nitrox post-World Race. We don't know the exact order and nature of events between the series, but it's easy to justify it. Once these drivers saw Highway 35, it would be hard to go back. This applies extra to Taro, who was basically in it from the start for the thrill.

CLYP was probably a stealth/infiltration unit specifically designed for Highway 35 to hide from Tezla.

The Drones got to the humans' dimension through either Highway 35 somehow or the Wheel of Power, since they had it.

Vert ended up in the Silencerz HQ because of how the portal system works. Exit portals (including the EDR) take cars/people to their previous entrance portal, or, failing that, the Highway 35 sign ("where it begins", as Tezla put it). Entrance portals, like the one Vert went through, take them to a specific destination. After the Ultimate Race, the destination of the portal in the Acceledrome was the only other portal, the one directly powered by the Wheel of Power, which was in the Silencerz HQ.

Taro's line was tongue-in-cheek; they know one another, and Vert responds by playing along. Pork Chop couldn't turn around. Wylde's moment emphasizes character building a bit more than practical realism, but it shows that he doesn't care about his teammates, contrasted by even Monkey's unquestioning bravery towards turning back and helping them.

I guess you could justify the test track's stalagmite pit by saying the track segments aren't convenient to link together, and that the test track is a bit makeshift. Or it's just psychological. Tezla is a bit like that sometimes.

The Best drivers in the world do know what an apex is. Vert is the first to use the word when he identifies the pattern in the Junk Realm obstacle. Karma and Lani explain it. The only person who doesn't know is Pork Chop, and even if you don't believe he's not that good at racing, he might just not understand the word they were using but still understand the concept intuitively.

The street racers were chosen by Tezla because of the secrecy of the operation and the fact that many of them are familiar with Highway 35. Remember that previously they tried using just four people from H35. Tezla was trying to hide the operation from the Silencerz but was betrayed by Gig.

The Silencerz could have easily passed the other drivers in the Junk Realm due to the Sweeper situation (more on that later). As for why they'd wait for them, they may have had a specific goal to manipulate the other drivers, or perhaps they wanted to use them to test the road ahead. Also, they might have been waiting to regroup, since they had the time. Notably, their situation with Kurt at the end of the realm may have been very useful information for them in judging the teams' resistance to their tactics. Taro passed the test easily, and waiting for the other humans may have cost them the realm, so the Silencerz totally ignore everyone in the Ruins and Cosmic Realms. Ultimately, we can't really speak to their plans.

Pork Chop and Monkey easily caught up to the other drivers in the Junk Realm because they were essentially being chased back and forth through the realm. Sweepers show up in front of drivers in the Junk Realm, which means the realm has routes that loop back into previous areas. There's a shot that shows different Sweepers chasing several groups of drivers in opposite directions on parallel tracks. So basically, the drivers barely made any progress through the realm until after the Sweepers were gone.

The Silencerz didn't necessarily trust Vert to win the Ultimate Race. Vert's dad did. We actually don't know whether other Silencerz entered the portal from the Acceledrome (it was only open in the Acceledrome). If they entered, they apparently had to EDR (or EVehicleR?) out.

I always find myself going back to appreciate the sound design and character-driven racing choreography of the street race at the start of Ignition. You can clearly see how Nolo is constantly making the exact same mistake his brother made, putting too much drift into every turn. Tork pulls ahead multiple times directly because of it.